Definitely Other.
Also, @Susanpoppin 's list is way too long and she picked Nanachi as sadder than Mitty for Made in Abyss and that's just wrong.
Sad backgrounds...
+ Menma from AnoHana
+ Kyo from Fruits Basket
+ Isla from Plastic Memories
+ Charce Lacroix from Astra: Lost in Space
+ Mitty from Made in Abyss
+ Tasuhiro Satou of Welcome to the NHK
+ Tomoko Kuroki from WATAMOTE
+ Violet Evergarden
+ Pretty much everyone in The Promised Neverland